The advantages of emulsions containing silicones having a large particle size, i.e., 1-100 micron (micrometer/.mu.m), is well documented in the patent literature, as for example in U.S. Pat. No. 5,302,658 (Apr. 12, 1994). While U.S. Pat. No. 5,302,658 provides for the production of emulsions containing large particle size silicone polymers, it does so by using surfactants to emulsify high viscosity silicone polymers which are difficult to handle and typically in short supply on a commercial scale. By way of contrast, the present invention in the most preferred embodiment, uses an anionic surfactant capable of functioning as an emulsifier to emulsify a silicone oligomer to a large particle size, and capable of functioning as an acid catalyst to polymerize the silicone oligomer, emulsified in the large size particles, to a high viscosity polymer.
These types of disadvantages are also present in U.S. Pat. No. 5,504,150 (Apr. 2, 1996), which while suggesting that a sulfonic acid catalyst can be employed, does not suggest the use of a dialkyl sulfonic acid as the catalyst. U.S. Pat. No. 5,504,150 also requires the use of expensive and complicated equipment including a reaction chamber, shearing and inversion devices, and collection vessels, to convert high molecular weight polymers to emulsions, in comparison to the present invention in which oligomers are first emulsified and then polymerized to higher molecular weight species using simple mechanical agitation.
While U.S. Pat. No. 3,294,725 (Dec. 27, 1966) prepares emulsions containing silicone particles by emulsifying siloxane oligomers using an anionic or a nonionic surfactant, and then polymerizing the emulsified oligomer particles to siloxane polymers of higher viscosity using condensation specific acid catalysts, the catalysts are not dialkyl sulfonic acids. In addition, U.S. Pat. No. 3,294,725 is silent with regard to the particular size of the particles, and where reference is made to the particles, they are merely described as being of "extremely fine particle size" or "so small that they cannot be resolved under an optical microscope".
Thus, U.S. Pat. No. 3,294,725 teaches against preparing an emulsion as contemplated herein, wherein the particle size is at least one micron (.mu.n) in diameter, preferably greater than 10 micron (.mu.m) in diameter. In fact, U.S. Pat. No. 3,294,725 teaches that a small particle size of siloxane reactant, i.e., high surface area, is desirable to get reasonable reaction rates.
Furthermore, it is known that the condensation polymerization rate of an emulsified siloxane oligomer is proportional to the total surface area of the siloxane oligomer particles in water, and therefore inversely related to the particle size. For example, reference may be had to the Journal of Polymer Science, Volume 20, Pages 3351-3368, (1982). Because of this, condensation polymerization rates are too slow to be practical in emulsions of siloxane oligomers having a particle size greater than about one micron (.mu.).
By way of contrast, the present invention is not limited according to such conventional wisdom, as it is believed that the condensation polymerization reaction of the siloxane oligomer occurs inside the particle rather than at the surface. Therefore, fast condensation polymerization rates have been observed according to this invention, and the rate does not decrease with increasing size of the particles.
Similarly, U.S. Pat. No. 4,990,555 (Feb. 5, 1991) fails to teach the use of siloxane oligomers of large particle size, and is further limited to the extent that the amount of water allowed in the process is taught to be less than one percent by weight. The process of the present invention by way of comparison is enabled to use 10-90 times as much water, without suffering any of the disadvantages noted in U.S. Pat. No. 4,990,555.
Thus, U.S. Pat. No. 4,990,555 teaches that the water concentration must be less than one percent by weight in order to obtain fast polymerization rates. In contrast, the polymerization rate in the process according to the present invention does not depend heavily on the water concentration over a very broad range, i.e., 10-90 percent by weight. The reason is that in the present process, an oil-in-water emulsion is formed in which polymerization occurs within and at the surface of the emulsion particles.
Furthermore, the process taught in U.S. Pat. No. 4,990,555 is directed to the bulk polymerization of siloxanes, and is not adapted to function for preparing an emulsion. This is because it is nearly impossible, as a practical matter, to prepare an emulsion in which an oil phase is emulsified in less than one percent by weight of water. The advantage of the present invention, therefore, resides in the ability to carry out the polymerization in water, and this eliminates the common handling problems frequently associated with polymers of high viscosity, i.e., molecular weight. For example, in an emulsion, the viscosity of the polymer forming the dispersed particles has a negligible effect on the viscosity of the emulsion itself. Thus, it is possible to prepare an emulsion containing for example, 50 percent by weight of a one million centipoise (mPa.multidot.s) polymer, that has a viscosity of only about 10-100 centipoise (mPa.multidot.s). By this means, handling and pumping of high viscosity polymers can be readily facilitated.